Abstract : An atmospheric pressure chemical ionization mass spectrometer was modified to measure either ion or neutral effluents from a jet engine. The instrument was set up behind an F-15 aircraft at Eglin Air Force Base and measured the ions formed in both the positive and negative spectrum. Since the ion concentrations in the jet plume were quite small and measurement times relatively short, most of the measurements performed were of ions artificially produced in the exhaust using a corona ion source. The most obvious change in the ion spectrum after the jet engines were started was the dramatic increase in what appeared to be sulfuric acid. The spectra resulting from these measurements are shown and discussed. (jg)